Spark plugs are universally used in internal combustion engines to ignite compressed fuel-air mixture in the cylinders of internal combustion engines. The spark plug includes a pair of spaced electrodes, between which a high-potential electric current is passed from the secondary winding of the ignition coil to produce an ignition initiating spark across the gap between the electrodes.
In an attempt to improve combustion efficiency, efforts have been directed to strengthen or intensify the sparking at the spark plug gap. The expected results of the intensified sparking is almost perfect combustion of the fuel charge taking place in the engine cylinders, which in turn contributes to improved engine output and cleaner combustion or exhaust gases. The latter contribution is particularly advantageous nowadays when strict antipollution control is being imposed on automobile emission gases throughout the world. One prior attempt to strengthen the sparking at the spark plug gap is to introduce a capacitive device into the firing circuit of the ignition system. The capacitive device provides added electric energy across the spark plug gap resulting in intensified spark discharge thereacross. Baur U.S. Pat. No. 3,683,232 proposes to form a capacitor of high capacitance connected in parallel to the spark plug gap by fitting the spark plug cap on the spark plug. The capacitor includes the widened electrode 15 and the conductive screening shell 16 placed, respectively, within and around the plastic insulating body 12 which serves as a dielectric separating two electrodes. However, the spark plug cap of the aforementioned Baur U.S. patent has many disadvantages which make it extremely difficult to obtain the desired results. To mention a few, for example, the plastic insulating body of the cap fitted on the spark plug undergoes earlier deterioration under high temperature heat and severe vibration generated in the engine, which in turn causes a decrease in the capacitance of the capacitor. Since the capacitance has a critical effect on the strength of the spark discharge across the plug gap, the earlier decrease of the capacitance is apparently a detrimental factor contributing toward unacceptability of the Baur spark plug cap. The fact that the spark plug cap in the fitted condition is inevitably put to severe heat and vibration also poses other problems for the plug cap capacitor. For one thing, the capacitor electrodes, especially the conductive screening shell tends to be brought out of intimate engagement with the outer surrounding wall of the insulating body forming more or less an air gap between the screening shell electrode and the dielectric body. For another thing, there is a strong possibility of the plastic insulating body being broken apart. These are also the factors that prevent the Baur cap from being universally employed on the spark plugs since in either of the above cases the plug cap capacitor gets out of order. In short, the Baur spark plug cap is fragile in nature and lacks mechanical as well as electrical stability which is essential for the cap to be used on heat and vibration transmitting spark plugs. In addition, the capacitor device of the Baur cap is bulky in structure and is not effectively protected against moisture and heat.